103 CELLULAR TISSUE. 



Sfxt. 2 2. Of generally distributed, incombustible constituents of the membrane, 

 compounds of Silicon, Calcium oxalate and carbonate are often contained in the 

 epidermis in remarkable quantity and form. 



Presence of silica, at silicification, is observed specially in epidermal layers, and is 

 particularly abundant in the cuticularised outer walls. Highly silicified epidermal 

 layers are characterised by hardness and firmness. Equisetum hiemale, Calamus 

 spec, Gramina, leaves of Ficus sycomorus, F. trachyphylla, Deutzla scabra, Celtis, 

 Ulmus, Davilla brasiliana, Parinarium senegalense. Magnolia grandiflora : a definite 

 relation however does not exist between silicification and hardness: the hard epidermis 

 of the leaves in most Palms, Mahonia aquifolium, Drimys Winteri, Rhododendron, 

 Hakea spec, Phormium tenax, the phylloclades of Ruscus aculeatus, and in Cycas 

 revoluta, has no silica. (Mohl.) 



Calcium oxalate is observed in the form of granules, or obvious crystals, in the 

 epidermis, especially in the cuticular-layers in the leaves of Welwitscbia, of many 

 Cupressinese, and Taxinese, in species of Ephedra, the leaves of Dracaena reflexa, 

 arborea, Draco, umbraculifera, Sempervivum calcareum Jord., and species of Mesem- 

 bryanthemum. When it occurs in large quantity it often gives to the epidermis a 

 dull white colouring, as in the above-mentioned Sempervivum, Mesembryanthemum 

 lacerum, incurvum, stramineum, Lehmanni, vulpinum, &c., and the white spots of the 

 leaves of M. tigrinum. 



Calcium carbonate is contained in great quantity in the membrane of many hairs, 

 and especially in the peculiar peg-shaped wall-thickenings known under the name of 

 Cjstolilhs, especially in the Urticacese and Acanthaceas. 



Silicon-containing epidermal layers have, according to Von Mohl's widely extended 

 investigations, been observed in species from 41 families of the most various large divi- 

 sions of the vegetable Ijingdom. 



The silica is found chiefly in the outer layers of the external wall: but Von Mohl 

 notes ^ that he had not observed it to be restricted to the outer cuticular lamella alone. 

 In most cases, when the epidermis is smooth and flat, the silicification extends over the 

 whole outer wall and the outer part of the lateral wall of the epidermal cells: ^.^. in 

 many grasses and Cyperaceae. More rarely, and only where the silicification is extreme, 

 the inner wall of the epidermal cells, and the sub-epidermal cells, which .border laterally 

 on the respiratory cavity (Deutzia scabra), take part in it. The guard-cells of the 

 stomata are silicified all round or partially. 



Partial silicification of the cell-wall occurs in other sohtary cases : thus it is limited to 

 the protruding knobs in the median line of the epidermal cells in the stem of Scirpus 

 palustris and mucronatus. On the subsidiary cells of Equisetum hiemale the transverse 

 bands on the lower wall are only silicified in their inner part which borders on the slit, 

 not in their outer part (Fig. 24, p. 72, A, Cj. Further there belong to this category the 

 stalks of the cystoliths in the Urticaceae, the silicification of which was proved by Payen, 

 and cystolith-like outgrowth of the wall of the above-named Borragineae and Compositse 

 (Onosma, Cerinthe, Helianthus trachelifolius, comp. p. 106). On the stinging hairs of 

 Urtica dioica the upper brittle part of the wall is very strongly silicified, the lower part 

 but little (Mohl, /.c, 219). 



Very often there are varieties in the silicification of cells and groups of cells of one 

 epidermal surface, due to the unequal extent of the silicification in closely neighbouring 



» Von Mohl, Botan. Zeitg. 1861, pp. 209, 305, where also the literature on the subject is 

 thoroughly treated of; and Hofmeister, Die Lehre von der Pflanzenzelle, p. 242. 



